1. Field of the Invention
This invention relates to novel photoresist compositions for forming positive resist images on a surface and to circuit fabrication using such novel compositions. The instant composition contains polymers having pendant acid labile groups in the presence of acid generators capable of forming a catalytic amount of strong acid upon exposure to actinic radiation.
2. Background of the Invention
Modern high resolution photoresists require high photospeed for productivity, superior image integrity, and fully aqueous development and stripping. All aqueous processing is needed because the organic solvents that have been used in the past are being replaced because of increasingly demanding environmental restrictions.
In U.S. Pat. No. 3,779,778, novel photosolubilizable compositions are disclosed that comprise (1) a water-insoluble compound containing one or more acid-degradable groups, and (2), a photoinitiator comprising a photolyzable acid progenitor. The useful water-insoluble compounds can contain one or more acid-degradable linkage which can be formed by the nucleophilic reaction of phenols, N-alkyl arylsulfonamides, or certain secondary amines with alkyl vinyl ethers, e.g. methyl vinyl ether, ethyl vinyl ether or a dihydropyran.
A resist prepared according to this invention from the THP ether of a phenolic resin and a photoacid generator required a mixture of flammable organic alcohol, potassium hydroxide and water to develop away the exposed areas.
European Patent Application 0,264,908 discloses resists comprised of at least one polymer, copolymer, or terpolymer having recurrent acid labile groups pendant to the polymer backbone wherein the improvement comprises: selecting a substituent side chain on said acid labile groups which is capable of forming secondary carbonium ion intermediates and having an available proton adjacent to the carbonium ion formed during cleavage. The autodecomposition temperature of the polymer comprising the photoresist is increased, to a temperature greater than about 60.degree. C., by selecting substituent side chains on the acid labile group which exhibit a less stable intermediate carbonium ion than the t-butyl ion.
In U.S. Pat. No. 4,491,628, resists sensitive to UV, electron beam and x-ray radiation with positive or negative tone upon proper choice of a developer are formulated from a polymer having recurrent pendant groups such as tert-butyl ester or tert-butyl carbonate groups that undergo efficient acidolysis with concomitant changes in polarity (solubility) together with a photoinitiator which generates acid upon radiolysis. A sensitizer component that alters wavelength sensitivity of the composition may also be added. The preferred acid labile pendant groups are tert-butyl esters of carboxylic acids and tert-butyl carbonates of phenols but, it is understood that a wide range of acid labile groups are operative in the invention. These include trityl, benzyl, benzhydryl modifications as well as others well known in the art.
These resists require that the exposed image be heated to 100.degree. C. or more to catalytically decompose the acid labile esters in the exposed areas. This heating creates a risk of image distortion if the glass transition of the coating is less than 100.degree. C.
The production of printed circuits using a variety of resists is described in "Printed Circuits Handbook" edited by Clyde F. Coombs, Second Edition, published by McGraw-Hill, Inc., 1979. Chapter 6 of this publication describes the use of both negative-working and positive-working photoresists in the preparation of printed circuits. In particular, section 14 of this chapter discusses conventional positive-working liquid photoresists and section 16 discusses conventional coating methods for applying such photoresists to printed circuit substrates. Likewise, the production of printed circuits using photoresists is described in "Photoresist Materials and Processes" by W. S. DeForest published by McGraw-Hill Book Company, 1975. In particular, Chapter 5 of this publication discusses conventional positive resists and the processes of their use to manufacture printed circuits.
The positive acting thick film liquid resist compositions of the current invention are fully aqueous developable, have high photospeed, do not require the use of high temperature bakes to solubilize the exposed image, and give high resolution images of good line quality. Properly formulated, the resist image can withstand both acid and alkaline plating and etching processes used to modify the substrates to which the resists are adhered. The resist image can then be blank exposed and stripped in aqueous alkali or re-exposed imagewise and developed so that the substrate can be modified a second time without the need to apply a second resist.
The resist compositions of the instant invention contain thermally stable but acid labile .alpha.-alkoxyalkyl esters of carboxylic acids, e.g., tetrahydropyranyl esters.
J. E. Kearns et al., J. Macromol. Sci.-Chem. A8(4), pp. 673-685 (1974) describe the preparation and deesterification of a number of polymers and copolymers of unsaturated tetrahydropyranyl esters. The utility of dihydropyran as a protecting group in the preparation of polymers containing other groups susceptible to normal hydrolysis conditions is ascribed to the mild conditions required to remove the tetrahydropyranyl group from these esters.
None of the prior art of which we are aware shows the use of the polymers of the present invention in thick film liquid photoresist compositions. Moreover, none of the prior art of which we are aware discloses resist formulations usable in thick film liquid photoresists containing a polymer bearing recurring pendant acid labile groups that possess the structures disclosed in the present invention.